Abstract
Aspergillus niger van Tieghem LPM 93 was shown in an earlier study to produce the most thermostable β-xylanase, which was effective for improving brightness and delignification of non-delignified and oxygen-bleached samples of eucalyptus kraft pulp. Here, we report the production, purification, and characterization of a xylan-degrading enzyme (XynI) from this strain grown in submerged liquid cultivation on medium containing sugar cane bagasse as the carbon source. XynI was isolated by ultrafiltration and gel-filtration chromatography and characterized. The fungus displayed high levels of xylanolytic activity after the second day of cultivation, and this activity remained constant up to the 50th day. The molecular mass of XynI was in the range of 32–33 kDa as determined by mass spectrometry and SDS-PAGE. The two-dimensional gel electrophoresis analysis showed the existence of multiple forms of β-xylanases in XynI. XynI showed the highest activity at 50°C and pH 4.5 and was stable in sodium acetate buffer at pH 4.5. The Km and Vmax values were 47.08 mg/ml and 3.02 IU/ml, respectively. Salts inhibited the activity of XynI to different degrees. N-Bromosuccinimide caused marked inhibition of XynI. On the other hand, β-mercaptoethanol and l-tryptophan were the best enzyme activators.
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